Chemisorption and reaction of ethylene on chemically modified Ru(001) surfaces

The adsorption and reaction of ethylene on a Ru(001) surface on which ordered p(2x2) and p(1x2) overlayers of oxygen adatoms are present have been investigated using high-resolution electron energy loss spectroscopy, thermal desorption mass spectrometry, and low-energy electron diffraction. In contrast to the di-sigma-bonded ethylene that is observed on clean Ru(001), ethylene chemisorbs molecularly in a ..pi..-bonded configuration at temperatures below 200 K on both the Ru(001)-(2x2)O and Ru(001)-p(1x2)O surfaces. All of the ethylene that is chemisorbed on Ru(001)-p(1x2)O desorbs reversibly at 160 and 240 K, whereas approximately one-third of the ethylene on the Ru(001)-p(2x2)O surface desorbs molecularly at these temperatures. Upon annealing to 250 K, the irreversibly adsorbed ethylene on the Ru(001)-p(2x2)O surface dehydrogenates to ethylidyne (CCH/sub 3/), which dehydrogenates further to vinylidene (CCH/sub 2/) below 350 K. This represents the first unambiguous identification of a surface vinylidene species, as well as the first isolation of an intermediate in the decomposition of surface ethylidyne that preserves carbon-carbon bonding. The vinylidene decomposes to adsorbed carbon and methylidyne (CH) below 400 K, and the methylidyne decomposes with the evolution of hydrogen between 500 and 700 K.